decided to keep trying different combinations/
number of points until I got a good solution.
We need to get the coordinates of our GCPs.
Google Earth uses the WGS84 datum, but you can
switch the format of the displayed coordinates to be
more convenient. Open the preferences pane of the
application and select your desired coordinates in the
“Show Lat/Long” area; you can also change to metric
units so elevation will be shown in meters (Figure
12). I chose to work in the UTM coordinate system
and know that I’m in UTM 15N. If you have lat/long
points, a website such as the Montana State
University geographic unit converter (www.rcn.
montana.edu/Resources/Converter.aspx) can do
a great job of helping out to get to whichever
coordinate system you’d like (Figure 13).
Now that Google Earth is showing us useful
coordinates, move your mouse over each GCP and
write down the coordinates from the bottom right of
the window (Figure 14). As you zoom in, the
program uses a pseudo perspective zoom technique.
Hit the “r” key to rectify the view back to a nadir
Next, locate at least three photos that show each
GCP. Open QGIS and click the “Add Raster Layer”
button in the left menu (see last month for detailed
instructions). Select the photo you want to open and
click OK on the next popup box. Now you can zoom
around on the photo and find your control points.
In the bottom of the window, you’ll see the
coordinates of the mouse cursor (Figure 15). Write
down the coordinates of each GCP to the nearest
integer value. Ignore the negative sign on the y
coordinate. Be sure to note which GCP and photo
name goes with which coordinates.
SERVO 05.2017 17
Figure 12: In the preferences pane of Google Earth, set the Show
Lat/Long setting to Universal Transverse Mercator and the Units
of Measurement setting to Meters, Kilometers.
Figure 15: Locate each GCP in multiple images using QGIS. Write down the
coordinates shown in the lower tool bar, ignoring any sign on the numbers.
Figure 13: If you need to convert between coordinate systems
and datums, look no further than the Montana State University
Figure 14: Place your cursor over the ground control point and note the
coordinates in the lower right corner of the window. Repeat this process
for each GCP.